[PATCHv4 2/3] powerpc, cpu: handling the special case when boot_cpuid greater than nr_cpus

[Pingfan Liu <kernelfans@xxxxxxxxx>]

For kexec -p, after boot_cpuid is mapping into the range of [0,
threads_per_core), then if nr_cpus is small, we will have the bitmap
[0,..., nr_cpus, ..., boot_cpuid, ...). This patch chooses cpus inside
the range of [boot_cpuid - nr_cpus +1, ..., boot_cpuid] to be online.

With this patch and the next, on a P9 machine with thread_per_core=4,
and set nr_cpus=2 for the crash kernel.
After
    taskset -c 11 sh -c "echo c > /proc/sysrq-trigger"
Then
    kdump:/sys/devices/system/cpu# cat possible
    2-3
    kdump:/sys/devices/system/cpu# cat present
    2-3
    kdump:/sys/devices/system/cpu# cat online
    2-3

Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
---
 arch/powerpc/kernel/setup-common.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/setup-common.c b/arch/powerpc/kernel/setup-common.c
index 1a67344..6920b5e 100644
--- a/arch/powerpc/kernel/setup-common.c
+++ b/arch/powerpc/kernel/setup-common.c
@@ -462,6 +462,8 @@ void __init smp_setup_cpu_maps(void)
 	struct device_node *dn;
 	struct device_node *boot_dn = NULL;
 	bool handling_bootdn = true;
+	int head_thread = 0;
+	int online_cnt = 0;
 	int cpu = 0;
 	int nthreads = 1;
 
@@ -499,13 +501,19 @@ void __init smp_setup_cpu_maps(void)
 			if (boot_cpuid < nthreads &&
 				be32_to_cpu(intserv[boot_cpuid]) == boot_cpuhwid) {
 				boot_dn = dn;
+				/* choose a bunch of continous threads */
+				if (boot_cpuid > nr_cpu_ids - 1) {
+					head_thread = boot_cpuid - nr_cpu_ids + 1;
+					/* keep the mapping of logical and thread */
+					cpu = head_thread;
+				}
 			}
 			if (boot_dn == NULL)
 				continue;
 		} else if (dn == boot_dn)
 			continue;
 
-		for (j = 0; j < nthreads && cpu < nr_cpu_ids; j++) {
+		for (j = head_thread; j < nthreads && online_cnt < nr_cpu_ids; j++) {
 			bool avail;
 
 			DBG("    thread %d -> cpu %d (hard id %d)\n",
@@ -520,13 +528,15 @@ void __init smp_setup_cpu_maps(void)
 			set_hard_smp_processor_id(cpu, be32_to_cpu(intserv[j]));
 			set_cpu_possible(cpu, true);
 			cpu++;
+			online_cnt++;
 		}
 
-		if (cpu >= nr_cpu_ids) {
+		if (online_cnt >= nr_cpu_ids) {
 			of_node_put(dn);
 			break;
 		}
 		if (handling_bootdn) {
+			head_thread = 0;
 			handling_bootdn = false;
 			goto again;
 		}
-- 
2.7.4


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